As is well known, it is customary, in many situations to place electrical conductors in a metallic conduit. Lengths of conduit may be connected together and/or formed, as may be required, to provide the effect of continuous conduit over extended lengths. A wide variety of coupling and angle changing fittings are provided, as may be required, to satisfy a wide variety of circumstances. In some construction situations, it is possible for two pieces of conduit to experience relative motion in a longitudinal direction. Unless special provisions are made, and unless the relative motions are very small, such motion may damage the conduit and/or fittings to which they are attached. Some of the more common and/or obvious situations wherein relative longitudinal motion may occur are on bridges which may rise and fall with temperature and load variations; interconnections between two buildings which may sway in response to wind loading; and situations wherein the ambient temperature of the conduit may be exposed to wide variations. Other situations will readily occur to those experienced in the art.
The industry has provided a variety of expansion fittings for use in these and other circumstances. For example, reference may be had to U.S. Pat. No. 3,154,632 issued Oct. 27, 1964 to F. L. Browne; U.S. Pat. No. 3,783,178 issued Jan. 1, 1974 to Robert A. Philibert, et al. and U.S. Pat. No. 4,250,350 issued Feb. 10, 1981 to Robert F. Polimine, all of which are assigned to the present assignee. Typically, the expansion fittings of the type described in the aforementioned patents will accommodate relative conduit movement of at least plus or minus four inches. The various cited patents, among other things, deal with means for providing superior grounding continuity between the two sections of conduit.
Another situation with which those familiar with the art are all too familiar are the dangers presented when the atmosphere within the conduit, and/or external to the conduit, may contain an explosive atmosphere. The explosive atmosphere may be unavoidable or unintentional and may result from various gases, vapor, dust, lint or other substances. As those familiar with the installation of electrical conduit are well aware, it is quite possible for the conduit to serve as a transmission path of an explosive atmosphere from one location to another. To prevent such transmission of explosive atmospheres, and/or to limit the propagation of products of combustion resulting from combustion and/or an explosion devices of the type indicated in U.S. Pat. No. 4,216,349 issued Aug. 5, 1980 to E. Wium have been designed. U.S. Pat. No. 3,974,933 issued Aug. 17, 1976 to Maria Toth, et al discloses another structure designed to contain and limit the effects of explosions ignited by electrical sparks.
Prior art expansion fittings provided the expansion feature but did not include means for preventing the escape of hot gases resulting from a explosion within the conduit.
There is at least one expansion fitting available in the market place that is designed for use in hazardous locations. This fitting is intended to prevent any exploded gases within the fitting from escaping at a tempearature which would ignite the atmosphere external to the fitting. The commercially available fittings, known to applicant, are relatively costly and permit a maximum expansion of only approximately 1.25 inches, an amount which is not comparable with the eight or more inches provided by the expansion fittings of the cited prior art patents. U.S. Pat. No. 2,900,436 issued Aug. 18, 1959 to A. I. Appleton describes this fitting. The patented fitting is not liquid tight and as a result, the critical surfaces may be exposed to corrosive atmospheres. In addition, the grounding technique described could damage the interior surface and permit rapid escape of hot gases.
Prior art techniques for confining hot gases and/or providing for relatively large longitudinal movement includes formation of a large "U" bend in the conduit or perhaps the use of a full circle. These techniques require costly flexible couplings.
Another technique was to use an expansion fitting of the type disclosed in one of the cited patents and near each end thereof provided a seal similar to that of the cited Wium patent. This combination is fraught with danger as longitudinal movement of the conduits could result in one or both of fractured seals and rupture of the conductor due to tension.